Recently, in various nations, in order to cope with environmental pollution, soaring oil price hikes and the like, the development of clean energy as an alternative to oil has been promoted. In parallel to this, measures to reduce oil consumption have been studied in a wide range of fields. As a part of those studies, in the automotive field, electric vehicles and hybrid electric vehicles utilizing onboard batteries have been developed and some of the hybrid electric vehicles have already been marketed today. As for the batteries to be mounted on such electric vehicles and hybrid electric vehicles, the development of a lithium ion nonaqueous electrolyte secondary battery, hereinafter referred to as a lithium ion secondary battery, which has higher battery performance and reliability compared to other batteries, has been promoted.
Many lithium ion secondary batteries use a carbonaceous material such as graphite, which is capable of absorbing and desorbing lithium ions as a negative electrode active material, and a lithium transition metal composite oxide, which is also capable of absorbing and desorbing lithium ions and is represented by a formula LixMyO2 (where M is at least one of Co, Ni and Mn), more specifically, LiCoO2, LiNiO2, LiNixCo1−xO2 (where x is 0.01 to 0.99), LiMnO2, LiMn2O4, and LiCoxMnxNizO2 (where x+y+z=1), or LiFePO4 and the like of a single kind or a mixture of two or more kinds as a positive electrode active material.
In devices that use the lithium ion secondary battery of this kind, for the purpose of higher space efficiency and heat dissipation, many use a prismatic battery in which power generating elements are housed inside a prismatic battery outer can. Such a prismatic lithium ion secondary battery is structured with an electrode assembly in which a positive electrode plate and a negative electrode plate with a separator interposed therebetween are spirally wound or stacked in a flattened manner being housed inside a prismatic battery outer can and a mouth portion of the prismatic battery outer can being sealed with a sealing cover. In the flat electrode assembly, both the positive electrode plate and the negative electrode plate are electrically insulated from the prismatic battery outer can and are electrically coupled, through the respective collectors, with a positive terminal and a negative terminal that are mounted on the sealing cover via an insulator.
The prismatic lithium ion second battery is fabricated by inserting the flat electrode assembly inside the prismatic battery outer can, then laser welding the sealing cover to the mouth portion of the prismatic battery outer can, subsequently pouring a nonaqueous electrolyte therein through an electrolyte pour hole and, thereafter, sealing the electrolyte pour hole. Such sealing methods by laser welding broadly fall into two categories: a method for laser welding by beaming a laser beam and jetting shielding gas simultaneously from a working head, and a method for laser welding by separating laser beam supply means and shielding gas supply means and scanning the laser beam single-handedly. The shielding gas is for the removal of oxygen from a welding point to create a working gas atmosphere suitable for laser welding, and an inert gas such as nitrogen gas and argon gas is used.
FIG. 9A is a perspective view schematically showing a laser welding device of the prior art employing a former method, and FIG. 9B is a cross-sectional view taken along the line IXB-IXB shown in FIG. 9A.
This laser welding device 100, as shown in FIG. 9A, has a working head 101, a pair of first and second sealing jigs 102A and 102B that clasp a prismatic battery B. The first and second sealing jigs 102A and 102B clasp both wide sidewall surfaces of a prismatic battery outer can B1 fitted with a sealing cover B2 at a mouth portion in an upper portion of the outer can B1, and position and secure the prismatic battery outer can B1 in a predetermined position. In the working head 101, one end of a cylindrical and hollow body of a predetermined length is tapered and, at the tip of the tapered end, an outlet of a predetermined size for a laser beam and shielding gas is provided. The other end of the cylindrical body of the working head 101 is coupled with a laser oscillator and a shielding gas supply source via flexible supply pipes not shown in the drawing. In the laser welding devices of this type, the laser oscillator used is typically a pulsed oscillator. The working head 101 is operated with an X-Y table and is adapted to move the outlet of the working head 101 to the welding point of the mouth portion of the prismatic battery outer can B1 and the sealing cover B2 and to simultaneously output a laser beam LB and a shielding gas N2 from the outlet to weld.
FIG. 10 is a perspective view schematically showing a laser welding device employing the latter method, i.e., the welding method by separating the laser beam and shielding gas.
In this laser welding device 103, the shielding gas is not supplied from the working head and is supplied via a separate route to a boxed container 104 of a predetermined size in which the prismatic battery is to be contained. The container 104 is a bottomed container in which the prismatic battery and a predetermined amount of gas are to be contained, and has an opening 106 formed at an upper wall surface thereof exposing the sealing cover B2 portion of the prismatic battery and provided with gaps formed to jet the gas from the circumference of the sealing cover B2. The container 104 has a gas supply inlet 105 formed on one sidewall, and the gas supply outlet is coupled with a gas supply pipe not shown in the drawing. In the laser welding device 103, the prismatic battery outer can B1 fitted with the sealing cover B2 in the mouth portion in advance is contained in the boxed container 104 and laser welding is carried out by supplying the shielding gas from the gas supply inlet 105 and scanning the laser beam LB from a laser head not shown.